import java.util.Arrays;

public class MyStack2<E> {

    public Object[] elem;
    public int usedSize;

    public MyStack2() {
        this.elem=new Object[10];
    }


    public void push(E val) {
        if(isFull()) {
            //扩容
            elem= Arrays.copyOf(elem,2*elem.length);
        }
        elem[usedSize]=val;
        usedSize++;
    }

    public boolean isFull() {
        return usedSize==elem.length;
    }

    public E pop() {
        if(empty())return null;
        E remove=(E)elem[usedSize-1];
        usedSize--;
        return remove;
    }

    public boolean empty() {
        return usedSize==0;
    }

    public E peek() {
        if(empty())return null;
        E remove=(E)elem[usedSize-1];
        return remove;
    }
}
